Background: Seroepidemiologic studies have provided information on the interaction of Chlamydia pneumoniae with other known risk factors of coronary heart disease. C pneumoniae infection appears to be more common in smokers than in nonsmokers, suggesting that smoking predisposes to the development of chronic C pneumoniae infection.
Methods and results: In identical twins, C pneumoniae-specific immunoglobulin A levels were found to be higher and cell-mediated immunity, measured as a lymphoproliferation response, lower in smoking twins than in their nonsmoking counterparts, suggesting that chronic C pneumoniae infections are common in smokers whose cell-mediated protective immunity appears to be lowered. Infections also may have an effect on lipid metabolism. Thus even in acute pneumonia caused by C pneumoniae, high-density lipoprotein (HDL) values are lower and triglyceride values higher than in pneumonia caused by viruses and other bacteria. Furthermore, chronic C pneumoniae infection has been associated with elevated triglyceride and lowered HDL levels in otherwise healthy Finnish men. We also have explored the combined effect of chronic C pneumoniae infection and markers of the metabolic syndrome (elevated body mass index, blood glucose and systolic blood pressure, and lowered HDL cholesterol) on the risk of cardiac events. The results suggest that chronic C pneumoniae infection enhances the effect of the metabolic syndrome on the risk of coronary heart disease.
Conclusions: The known risk factors of coronary heart disease may be explained partly by their interaction with chronic C pneumoniae infection. Further studies are needed to elucidate the pathogenetic mechanisms underlying these interactions.